4 numbers to watch for in Twitter's SEC filing

It looks like Twitter could file its public S-1 as early as Thursday afternoon. Sources told CNBC that Twitter sent an email out Tuesday to a group of former employees whose shares are covered by a 180-day lockup period after the company goes public.

Attached to the email was a form from Goldman Sachs for them to fill out confirming that they're indeed part of that shareholder group. They're asked to return the form by 5 p.m. PDT Wednesday, so if the bankers need a day to tally the forms, that means a public S-1 could hit Thursday.

That filing will reveal all the information investors have been dying to see. Of course, we'll see the size of its planned initial public offering, but the most influential statistics will be about the company's growth.

The key metrics to watch are revenue, profits and active user numbers, and the growth rate of those three. These numbers will determine Twitter's ultimate valuation, and how appealing the company is to investors.

CNBC's Julia Boorstin reports the social media giant, Twitter, has filed for an IPO.

Twitter is expected to bring in $582 million in ad revenue this year, up from $288 million last year. But it's not just about how big Twitter's revenue is—it's where that revenue is coming from, compared to where its users are.

Twitter's users are expected to be pretty diverse, from all over the world, but just a tiny percentage of its revenue is expected to come from outside the U.S.—eMarketer expects 90 percent of Twitter's revenue to be from the U.S. last year, and 83 percent this year. The heavy dependence on U.S. revenue signals a huge opportunity to grow revenue overseas—but also a significant challenge.

It raises the question of whether Twitter will be able to sell ads on the same scale—and at the same margins—outside the U.S., as it does here.

And for comparison, investors are sure to look back at how Twitter's key numbers compare to Facebook's, back when it filed its first S-1 in February 2012. There's no question that Twitter, with a valuation estimated between $10 billion and $15 billion, is expected to be far smaller than Facebook, which had net profit of $1 billion in 2011.

The question is whether Twitter's growth is anywhere near as fast as Facebook's—its revenue grew 88 percent from 2010 to $3.7 billion in 2011.

But it's not just numbers—Twitter's S-1 filing will hold a lot of valuable qualitative data, most notably, where Twitter sees its biggest risks and opportunities. In its initial S-1 filing for its IPO, Facebook said its biggest risk factor was increased mobile engagement "because ads and commercial content are not displayed on Facebook mobile apps."

That won't be a problem for Twitter, but it could raise questions about lack of mainstream appeal (just 18 percent of American adults use Twitter), advertising alienating users, spam, hackers, and of course competition. The social media landscape is fragmented, with increasingly popular new communication tools, including Snapchat, Instagram, and Whatsapp. And then there's always the next big app that hasn't launched yet.

Along with risk factors, Twitter will describe its strategy and how big the it believes its potential market is. Facebook said in its S-1: "Our addressable market opportunity includes portions of many existing advertising markets, including the traditional offline branded advertising, online display advertising, online performance-based advertising, and mobile advertising markets."

Twitter didn't point to a specific market size, just noting that total worldwide ad spend was $588 billion in 2010.

Plus, we'll see who Twitter's largest shareholders are. With a lot of management upheaval in its early years, we could see former CEO Evan Williams and co-founder Biz Stone holding significant stakes. CEO Dick Costolo, who was an early investor before he joined the company, plus chairman and co-founder Jack Dorsey are also sure to own significant stakes. Also, we will see how much Twitter's executive ranks are paid.

And then there's Twitter's stock ticker. TWTR seems like the likely pick.